Research On The Interface Microstructure,Thermodynamics And Kinetics Of Internal Oxidized Ag-Sn Alloy

As the most important substitute for the replacing of conventional toxic Ag-CdO contact material, the electric contact stability of Ag-SnO2was highly affected by its interface structure. However, the interface structure and its properties have not been deeply and comprehensively investigated till now. It is important for us to insight the interrelationship between the interface structure, chemical composition, processing parameters and its properties, which was helpful to the improvement of this material, including the structure optimization, processing improvment and properties prediction.Based on the investigation of microstructure characterization by a combination of metallographical microscope (OM), X-ray diffraction (XRD), Electron probe microanalysis (EPMA) and transmission electron microscope (TEM), the internal oxidation mechanism of Ag-3Sn-(In, Bi, La) alloy were disscussed in details; The interfacial structures and crystallographic orientation relationships of the Ag/SnO2interface in-situ formed in internal oxidized Ag-3Sn alloy were characterized; With the first principle thermodynamic calculation, the surface properties of cubic metal (Ag) and rutile-type oxide (SnO2, RuO2), and the interface properties of Ag/SnO2interface were assessed. Combined with interface structure characterization, interfacial properties thermodynamic assessment and the internal oxidation diffusion kinetics, a modeling of the thermodynamic and kinetics of internal oxidized interface in Ag-Sn alloy was proposed, by which the interrelationship between composition, interface microstructure, processing parameters and properties of internal oxidized interface in Ag-Sn alloy was analyzed quantitatively. The main conclusions were presented as follows:(1) The SnO2particles formed in-situ in the internal oxidized Ag-3Sn alloy are mainly constituted by elongated octahedral shaped and plate shaped particles, with highly preferred interface orientation relationship of (111)Ag//(101)SnO2.The phase compositions of Ag-3Sn-In alloy are Ag, SnO2, In2O3. The In2O3and SnO2particles are always precipitate together. The phase compositions of Ag-3Sn-Bi alloy are Ag, SnO2and The SnO2and Bi2Sn2O7particles formed as wrapping structure. The phase compositions of Ag-3Sn-La alloy are Ag, SnO2and La2Sn2O7. The La2Sn207particles with large size formed at grain boundary, with small quantity of large SnO2particles. Large quantity of small SnO2particles with size of about～10nm formed and dispersed uniformly inside the grains.(2) The formation of oxidation band in Ag-3Sn alloy was controlled by a dynamic equilibrium between the concentration changes of oxygen and Sn atoms during the internal oxidation process. Enhence the diffusivity of oxygen or reduce the back-diffusion behaviour of Sn atoms is an alternative method to prevent the formation of oxidation band. With the addition of In, the back-diffusion behavour of Sn atoms was restained effectively by the priority nucleation of In2O3, which promote the precipitation of SnO2at the Ag/In2O3interface. The grains were refined markedly by the addition of Bi element and resulted in a large quantity of grain boundaries. Oxygen diffused quickly along the grain boundaries and fully internal oxidation was processed. The diffusion of Bi atoms to Ag/SnO2interface was promoted by the priority nucleation of SnO2and reacted with SnO2to form Bi2Sn2O7particles. The oxygen in Ag-3Sn-La alloy is easily diffused along the phase boundary of Ag and eutectic La5Sn3, and reacts with La5Sn3to form La2Sn2O7. The dispersed SnO2particles with nano size were formed by the diffusion of oxygen from grain boundaries to the inside of grains.(3) The Ag/SnO2interface structure in internal oxidized Ag-3Sn alloy was characterized by a combination of TEM, HRTEM, and SAED. The in-situ formed SnO2precipitates highly preferred growing with interfacial orientation of (111)Ag//(101)SnO2,<110>//<010>SnO2Three kinds of orientation relatioships (ORs) between Ag matrix and SnO2precipitates were identified in the internal oxidized Ag-3at.%Sn alloy by using the matrix method and stereographic projection method: Type Ⅰ:[110]Ag//[010]SnO2,(111)Ag//(101)SnO2TypeⅡ:[112]Ag//[010]SnO2,(111),Ag//SnO2TypeⅢ:[110],//[001]SnO2,(111)Ag//SnO2, (4) The surface stabilities of low-index surfaces of cubic metal (Ag) and rutile-type oxide (SnO2, RuO2) were investigated by first-principles calculations. It was found that the ordering of Ag surface energies is:(111)<(100)<(110); the ordering of SnO2surface energies is:(110)<(100)<(101)<(001); the ordering of RuO2surface energies is:(110)<(101)<(100)<(001).(5) Combined the first principle calculation and surface thermodynamic analysis, the surface stabilities of these two oxides were evaluated as functions of oxygen partial pressure and temperature. And the "surface phase diagram" of these two oxides was constructed, which corresponds well with the experimental observations. The environment-dependent surface stabilities assessment of these oxides was look forward to the prediction of crystal morphology, by utilizing the Gibbs-WulfF theorem of equilibrium crystal shape (ECS). The ECS as well as the preferred growth morphology of these oxides was predicted.(6) Based on the experimentally characterization of Ag/SnO2interface structure, the interfacial properties, including the interface atoms relaxation and interfacial electronic structure, of Ag(111)/SnO2(101) interface were investigated by first principle calculations. The interfacial bonding strength of Ag(111)/SnO2(101) interface were assessed by the work of separation (Wsep) and the work of adhesion (Wad).The Ag(111)/SnO2(101) interface energy was presented as a function of temperature and oxygen partial pressure. With the combination of first principle calculation and thermodynamic analysis, a "interface phase diagram" of the Ag(111)/SnO2(101) interface was constructed firstly.(7) With the analysis of the internal oxidation kinetics of Ag-Sn alloy and the thermodynamics process of oxygen dissolved in silver alloy, the local oxygen partial pressure in internal oxidation region was expressed as a complex function of the environmental oxygen partial pressure, alloy composition, temperature, time and distance from the surface to the internal oxidation zone, by which the local oxygen pressure inside internal oxidized region was predicted.(8) Combined with interface structure characterization, interfacial properties thermodynamic assessment and the internal oxidation diffusion kinetics, a modeling of the thermodynamic and kinetics of internal oxidized interface in Ag-Sn alloy was proposed, by which the interrelationship between composition, interface microstructure, internal oxidation parameters and properties of internal oxidized interface in Ag-Sn alloy was analyzed quantitatively. The prediction of internal oxidized Ag/SnO2interface structure and the interfacial properties at any given conditions can be achieved. This modeling was implemented on the internal oxidation of Ag-3Sn alloy under air atmosphere, with oxidation temperature of1023K and823K, respectively.